What exactly is "hard surface modelling"?
There are many ways how to achieve the end-result shape of your object. Sometimes it's not only one way, but multiple ways are used based on how they are fit. We can distinguish these modelling techniques:


poly-modelling (extruding polygons from a primitive into complex shape)
spline-modelling (lofting curves into complex shape)
box-modelling (dividing a primitive into complex shape)
sculpting (controlling all surface points of complex shape with brushes)


The first 3 are parametric modelling techniques - you control limited amount of points to define the surface. You create curvature using Beveling or by Subdivision surfaces. Because of this, these techniques are useful for modelling fairly regular hard-surface shapes that do not change.

Because these techniques are parametric, it is easy to precisely define surfaces that need to be perfectly flat, need to have exact curvature or carefully placed creases. It is very fast to construct simple shapes.

On the other hand soft-surfaces, like organic tissue, are highly irregular. To model them would result in using too many control surface points. Because of this sculpting is the preferred technique here as it allows to quickly define irregular shapes.

Techniques used to create hard-surfaces have similar mindset of building control surface topology and they complement each other closer, so that's why such modelling is referred as hard-surface.

Sculpting is another skill-set to master, that is why it is usually separated.



In 2D graphics the analogy is vector vs raster - vector graphics allows for exact shape definition and raster allows to make complex shapes fast with brushes (sculpting in 3D is not exactly a raster technique, as that would need to be voxel sculpting, but we can honestly squint eyes to make the comparison).



My 2 cents on the matter:

It is unfortunate to split modelling between hard-surface and organic today, as now it is obsolete and has mostly historical reasons. Many sculpting packages are good with dealing with hard-surface with addition of specialized brushes and tools. Imho it makes better sense to distinguish techniques between sub-d (parametric) and sculpt (and if you want retopo, but topology knowledge comes intrinsic from sub-d and vice-versa) to be clear.